1. Field of the Invention
The invention relates to devices known as undulators which, in free electron lasers (FELs), enable the direct conversion of a part of the energy of a beam of relativistic electrons into light energy.
2. Description of the Prior Art
A free electron laser is constituted, as shown in FIG. 1, by:
a high energy electron source which is formed by an electron gun 1 followed by an induction or r.f. linear accelerator 2 which emits bursts of accelerated electrons bunched in packets of several thousands, one burst having the duration of a micropulse of several tens of picoseconds and one packet having the duration of a macropulse of several tens of microseconds;
a first magnetic deflection system 3 enabling the path of the accelerated electrons to be deflected towards a tuned laser optical cavity 4;
said tuned laser optical cavity 4 with its mirrors 5 and 6, and its optical window 7 for the output of the light waves;
an undulator 8 which is positioned in the median part of the optical cavity 4 on the path of the beam of accelerated electrons and generates a transverse magnetic field that is alternately polarized North-South/South-North along the direction of propagation of the electron beam, and
a second system of magnetic deflection 9 enabling the path of the electron beam to be deflected after it has passed into the optical cavity 4 so that it escapes laterally therefrom with respect to the optical window 7 for the output of the light waves.
The principle of operation of the free electron laser is based on the decelerating radiation that results from the fact that any electron undergoing acceleration or deceleration radiates electromagnetic energy and on the relativistic shortening of the wavelength.
In the undulator, the electrons crossing an alternatively polarized transverse magnetic field undergo, as shown in FIG. 2, deflections of small amplitudes, sometimes in one direction and sometimes in the other, and radiate light energy in the direction in which the the beam is propagated. In conditions close to synchronism between the forced oscillation of electrons and the light wave, there is a transfer of energy from the electrons to the light wave and the triggering of a laser phenomenon in the optical cavity.
The wavelength of the light emission may be written as follows: ##EQU1## .lambda.wave is the magnetic pitch or the pitch of the undulator .tau. is equal to: ##EQU2## v being the velocity of the electrons and c the velocity of light; K is equal to: EQU K=0.94 .lambda. wave Bo.
Bo being the value of the magnetic induction produced by the undulator.
It is observed that the wavelength of the light emission is all the shorter as the magnetic pitch wave of the undulator is small.
Undulators using this principle of physics have already been used in many devices.
They use magnetic circuits that are constituted either by a structure with permanent, electromagnetic or hybrid magnets or a structure without any magnet with an electrical conductor that decribes undulations on the path of the electron beam and is crossed by a strong excitation current.